Spinels are well known metal oxides of specific structural configuration having a generic formula M.sub.3 O.sub.4 where M represents the same or different metal elements having different valences whose sum of products of the valence times the number of atoms of each element having that valence equal, preferably eight, but may vary up to a few percent excess or deficiency of the metal ion relationship from eight. Exemplary of the common formulae are, e.g., MgAl.sub.2 O.sub.4, and ZnCo.sub.2 O.sub.4 where the sum of the product of the positive elements valence times the number of atoms of each valence equals eight. Exemplary of imbalanced stoichiometry, excess and deficient atom structures, are, e.g., Mg.sub.0.9 Fe.sub.0.11 Al.sub.2 O.sub.4 and Ni.sub.0.2 Co.sub.0.79 Co.sub.2 O.sub.4 respectively.
Most prior art techniques used commercially for preparing ceramic spinels employ the fusion technique of the metal oxides. This technique is not wholly satisfactory for the preparation of ceramic spinels because the metal atoms do not completely form into the spinel lattice structure; that is, some metal atoms form a segregated oxide phase admixed with the spinel lattice structure and once formed by fusion the crystals are not amenable to shaping by pressure and sintering without the aid of binders which may be detrimental to acid and/or base resistance and physical properties of the finished product. Organic binders in ceramics made in this way make the body relatively porous when they are removed during or after shaping. Segregated ceramic binders may weaken the body because they are the site of differential expansion and contraction and/or chemical attack.
The prior art also recognized the phenomena of spinel formation being a physio-chemical reaction based on thermal conditions such that, regardless of the ratio of the metals, some spinel lattice would form at the correct temperature, physical and chemical conditions, albeit those atoms not forming a spinel lattice structure remain as segregated phases of the metal oxides. The spinel shapes which are commercially available usually have been prepared from spinels produced from starting materials containing impurities or one or more segregated metal oxides phases and thus are relatively poor with respect to their physical properties, e.g., tensile strength, acid and/or base resistance and porosity.
Numerous patents and scientific literature have been published disclosing different techniques for preparing spinels (esp. MgAl.sub.2 O.sub.4). Most procedures employ metal oxides or oxidizable compounds, both of which are converted to a spinel by firing or fusion with or without pressure.
In some patents a magnesium compound and an aluminum compound are mixed to give the requisite molecular constitution, are wet ground and mixed, and fired at temperatures up to 3,000.degree. F. (ca 1660.degree. C.) as for example, in U.S. Pat. No. 2,618,566 or shaped before firing into pebbles as in U.S. Pat. No. 2,805,167.
Others use pure magnesia and alumina mixtures which are then fired at 2150.degree. C. and cooled slowly overnight, (e.g. U.S. Pat. No. 3,516,839). Still others mix alumina with magnesium nitrate, dry fire on a schedule to 1400.degree. C., and then grind to obtain a powder, (e.g. U.S. Pat. No. 3,530,209). Another technique follows the fusion route of magnesium nitrate hexahydrate and ammonium aluminum sulfate dodecahydrate (both reagent grade) to 1,300.degree. C. to produce a fine powder, (e.g. U.S. Pat. No. 3,531,308). A magnesium-salt (MgSO.sub.4.7H.sub.2 O), aluminum-salt (Al.sub.2 (SO.sub.4).sub.3.18H.sub.2 O) mixture, co-crystal has been employed to prepare a powder which is then shaped into ceramic bodies by hot press techniques with or without the use of binders, (e.g. U.S. Pat. No. 3,544,266).
Concomitant with these developments researchers investigated the nature of metal double hydroxides formed by coprecipitation, some of which were shown to convert to a spinel upon calcination. Early work was performed by Feitnecht and his students who made a series of double hydroxides with Mg/Al ratios of 2.5 to 1, even employing a reactant range of 1.5-4 to 1 Mg/Al, by coprecipitation from magnesium and aluminum chlorides, Helv. Chim Acta 25, 106-31 (1942), 27, 1495-1501 (1944). No change could be detected by x-ray diffraction techniques then available for different Mg/Al ratios or a certain degree of substitution by chloride for hydroxide. A similar double hydroxide, reported to be a hydrate even after heating to 150.degree. C., was reported by Cole and Hueber in "Silicates Industriels" Vol. 11, pp 75-85 (1975). The compound was made by the reaction of NaOH with Al metal or Al.sub.2 (SO.sub.4).sub.3 and MgO or MgSO.sub.4 at 65.degree.-70.degree. C. The product had a Mg/Al ratio of 4/1 even when reactant proportions were varied. However, Mg(OH).sub.2 was observed as a second phase in some cases.
More recently, Bratton in both Journal of The American Ceramic Society, Vol. 52, No. 8 (2969), and Ceramic Bulletin, 48, #8 pp 759-62 (1969) 48, 11, pp 1569-75, reported the coprecipitation of numerous magnesium and aluminium chlorides and oxalates which on heating, drying, calcining or firing, exhibited a spinel x-ray diffraction crystallographic pattern. The coprecipitation product resulted in a magnesium aluminum double hydroxide of composition 2Mg(OH).sub.2.Al(OH).sub.3, plus a large amount of segregated gibbsite Al(OH).sub.3 phase (see also U.S. Pat. No. 3,567,472). This is presumably the same product Feitnecht obtained.
Bakker and Lindsay in "Ceramic Bulletin" Vol. 46, No. 11, pp 1095-1097 (1967) report that a high density spinel body can be made from Mg(OH).sub.2 and Al(OH).sub.3 if 1.5% AlF.sub.3 is added as a mineralizer.
In the works cited above these powders were, in some instances, calcined then fired while in other instances the powders were heated through the calcining range and ultimately through the firing and even the fusion range. Early work was directed to preparing spinels usable as a decolorant, U.S. Pat. Nos. 2,395,931 and 2,413,184 or as antacids, U.S. Pat. Nos. 3,323,992 and 3,300,277. In the last case a "highly hydrated magnesium aluminate" is claimed as a new composition of matter, the formula of which is Mg(OH).sub.2.2Al(OH).sub.3.XH.sub.2 O where X=4 to 8. The material is prepared by the reaction of NaAlO.sub.2 (Na.sub.2 Al.sub.2 O.sub.4), NaOH and MgCl.sub.2 as aqueous solutions at a pH from 8-9. Bratton in U.S. Pat. No. 3,567,472 also discloses coprecipitation of a magnesium and aluminium chloride from a solution having a pH from 9.5 to 10, drying or firing to obtain a light-transmitting spinel by adding CaO.